1. Field of the Invention
The present invention relates to a method of producing a conductor pattern, or structure, on a substrate and in particular to a method for punching a conductive base material in order to obtain a patterned, or structured, conductor pattern connected to a substrate, where the conductor pattern particularly may be an antenna pattern, or structure, for an RFID label for an RFID system (RFID=radio frequency identification).
2. Description of the Related Art
Identification systems and particularly wireless identification systems such as barcode systems, OCR systems (OCR=optical character recognition) or RFID systems have become more and more widespread. Applications are for example identification systems especially for personal identification, animal identification, brand and product protection, logistics, automobile anti-theft devices, security locking systems etc.
RFID systems work in a frequency range of about one hundred kHz up to some 10 GHz. RFID systems are not affected by pollution or wear since no mechanical contacts are present, and, in contrast to barcode systems and OCR systems, do not need an optical connection. Moreover, several data carriers may be read simultaneously at high speed and, depending on the RFID system, the information on the data carrier may even be changed.
In general, RFID system are realized as transponder systems and essentially comprise two different components, one or several RFID transponders, also referred to as RFID data carrier, RFID label or RFID tag and e.g. are mounted on the objects to be identified, and an RFID base station which depending on the RFID system is configured to be either a reader or reader and writer and which may read out the data from the RFID transponder and may possibly change the data by means of write-in.
An RFID transponder in turn fundamentally consists of an integrated circuit that takes over the editing and processing for transmitting and receiving or respectively coding and decoding the data as well as all further functions, such as the storage of the identification number or the data encryption, and an antenna for wireless communication with the RFID base station. The integrated circuit of the RFID transponder is also referred to as RFID transponder chip or RFID transponder IC (IC=integrated circuit), the antenna is also referred to as RFID antenna pattern.
All components of the RFID transponder including the RFID antenna pattern are arranged on a substrate and protected from environmental influences by covering foils or a housing.
RFID transponder systems may be classified in a number of ways. An important distinguishing feature is the type of energy supply of the RFID transponder. Here a division takes place into passive and active systems.
An RFID transponder having an internal energy supply is described as active. The batteries (flexible flat cells, button cells, etc.) e.g. contained in active RFID transponders allow the additional operation of micro controllers for processing and storing comprehensive data. Thus the integration of sensors to measure temperature, pressure and shocks (impacts) is gaining increased importance. In this way, miniaturized data loggers for recording data may be realized which in the form of smart label of smart card present a low-cost alternative to established systems. Similar to smart cards, these RFID transponders may be supplemented with further elements such as displays or keyboards in order to increase their functionality.
Active RFID transponders are currently produced for transmission frequencies of 125 kHz, 135 kHz, 13.56 MHz, 433 MHz, 868 MHz, 915 MHz and 2.45 GHz, so that the ranges of active RFID transponders lie between several meters and up to 100 m.
An RFID transponder is referred to as passive when it is supplied with energy via an external predominantly magnetic or electromagnetic field. In most RFID systems the energy supply and data transmission takes place via an inductive, i.e. magnetic coupling between the RFID transponder the and RFID base station.
Due to the external energy supply of passive RFID transponders, their architecture may be made very simple and thus cost-effective. An RFID transponder based on inductive coupling mostly consists of only one RFID transponder chip which is connected to an antenna patterned on a substrate. This embodiment, in particular, is referred to as RFID label.
Most passive RFID systems work at frequencies of 125 to 135 KHz and 13.56 MHz. The range is limited to about 1.5 m. There are also other passive RFID systems that work at higher frequencies, e.g. 868 MHz or 915 MHz.
One field of application for RFID labels is the production of security packaging for highly expensive products. On the inside of security packaging, a copper grid or a copper strip may be found. The copper grid or the copper strip have a defined resistance. When the packaging is opened and the copper grid or the copper strip is torn, the resistance approaches infinity which the RFID transponder chip registers and saves along with the date and the time. With the help of this technology it is possible to precisely determine whether the product was removed from the packaging only at the customer's end or whether the product was stolen during transport or at the manufacturer's company. The simplest security packaging consists of an RFID antenna pattern glued across the opening area of the packaging. As soon as the packaging is torn open, the RFID antenna pattern is destroyed and the write/read function of the RFID label is thereby inactivated. The field of application for security packaging are high-value goods such as cell phones and medicine.
Further applications for RFID systems in the MHz and GHz ranges are the guarantee of counterfeit-proof identification cards, entrance tickets, authorization IDs and bills. In this field, the integration of a 2.4 GHz RFID transponder into a 200 Euros bill is currently under consideration. In a bill, a modified aluminum security thread which suffices for a working frequency of 2.4 GHz may serve as an antenna. The advantage of such RFID transponders that work at considerably more short-wave frequencies than 13.56 MHz is that a much more compact design of the antenna is possible. While at 13.56 MHz multi-loop RFID antenna patterns are still needed, a single loop suffices for 800 MHz and higher frequencies.
The mechanical structure and the quality, in particular the grade, of RFID antenna patterns are of considerable importance for the utilizability and the reliability of the entire RFID system. It is true that the size and the number of loops of the RFID antenna pattern may be reduced, however only with the disadvantage that performance and reliable data transmission will suffer. In much the same way, the conductivity and the resistivity of the materials of the RFID antenna pattern play decisive roles in terms of the grade of the coil. The lower the resistance of the material of the RFID antenna pattern, the higher the grade and the associated range and guarantee of an error-free data transmission between the RFID label and the RFID base station.
RFID label production may be subdivided into three production sections, i.e. producing the RFID transponder chip, producing an RFID antenna label consisting of the RFID antenna pattern comprising mostly copper or aluminum and consisting of a substrate material, and placing the transponder chip onto the RFID antenna pattern.
Producing RFID labels has so far been a costly and high-effort technology, wherein mostly very expensive methods and, thus, a very large number of production steps using a large number of different production machines have been needed. This results in very high investment costs for a continuous production plant for RFID labels. There are currently three relevant traditional approaches to RFID antenna production which are based on screen printing, etching or sputtering/electroplating techniques.
The chart below depicts a summary of the essential production steps of the above-mentioned production techniques. Following that, a detailed description will be given of the individual production techniques and their disadvantages or problems.
Screen-printingElectroplating/Etching techniquetechniqueSputteringcoil design using CADcoil designcoil design using CADmask fabricationusing CADsheet film fabricationcoating the copper tapesheet filmprinting silverwith photoresistfabricationconductor paste ontophotoresist exposureprinting silverPET tape or generallyphotoresist development +conductor pasteconductive materialcleaning of the surfaceonto PET tapedrying the silveretching copper + cleaningdrying theconductor pastesurfacesilver conductorgalvanic reinforcement +stripping copper + cleaningpastecleaning and dryingsurface
What follows now is a description of the etching technique. The etching technique necessitates a very large number of processing and/or production steps so as to pattern a foil, the whole area of which is clad, e.g., with copper. Because of the strong inclination of the copper to form surface oxidations, the top copper surface has a tarnish protection located thereon which is removed, prior to the etching operation, using a solvent or a cleaning brush, e.g. in the form of a pumice stonemeal brush. In the first step, the copper surface is coated with a photosensitive lacquer, for example by laminating on a solid resist, by curtain coating, spray coating or roller coating. Subsequently, the photoresist is exposed, using an exposure tool and a photo mask, in those areas where the photoresist is to protect the copper surface. This technique is also referred to as a negative-resist system, or as a negatively operating system. In this context, the photoresist is cross-linkable at a UV radiation of 350 nm. The third step comprises developing the photoresist within the developing unit, mostly using sodium carbonate as the developing solution. Here, the non-exposed areas are washed away from the copper surface and are thus accessible for the etching solution. Subsequently, the non-protected areas are etched away within the etching system, for example using iron trichloride (FeC13) or sodium peroxodisulfate (Na2S04). In the last step, the remaining photoresist is washed away, mostly using potassium hydroxide (KOH), and the surface is cleaned in a rinsing cascade with deionized water so as to neutralize any alkaline or acid surface contaminations to avoid a high level of surface oxidation.
However, these etching techniques have severe disadvantages or are problematic for the following reasons. A very large number of different machines are needed, for example, a laminator, an exposure unit, a developer, an etcher and a stripper, so that high investment cost is needed. In addition, wet-chemical processes are used wherein the waste water must be treated in a time-consuming and costly manner so as to remove the copper ions, which represent a hazard to the water, from the rinse water.
What follows is a description of the screen printing technique. In screen printing, a conductive paste, e.g. a silver conductor paste, is printed through a patterned screen onto the substrate material using a squeegee. The screen is produced using a sheet film and a screen exposure unit. The screen is permeable to the conductor paste in all those locations which are not covered by the screen coating, i.e. the photoresist. The conductive paste is cured in a drying section at a specific temperature from about 120 to 160° C.
Screen printing has several disadvantages, or is problematic for the following reasons. The silver conductor pastes are very expensive and additionally, the conductance of the RFID antenna pattern fabricated therewith is low, so that as a result, the grade and range of the RFID antenna pattern is relatively poor. In addition, the service life of such screens for screen printing techniques is short.
What follows is a description of sputtering. In sputtering by means of a shadow mask, an RFID antenna pattern may be fabricated directly on the surface of the substrate material. What is disadvantageous about this technology are the high costs of a sputtering system and, additionally, the high cost of operating and maintaining the system. A good conductive layer for an RFID antenna pattern may be fabricated only when a minimum concentration of foreign atoms is present within the sputtering system, which means, in turn, that a high vacuum must be generated, which is also compulsory for the sputtering process itself. However, despite the high expenditure, only very thin metal layers, typically within a range of several nm, can be fabricated using a sputtering process. This means that the inductance, or ability to carry current, of the RFID antenna pattern generated by means of sputtering is very small (as in the stamping technique to be described below). If a higher level current-carrying ability or a higher level of inductance is needed, the thin, sputtered RFID antenna pattern first needs to be reinforced in an additional production step by means of an expensive and wet-chemical electroplating process.
What follows is a description of stamping. In stamping, a very thin copper, aluminum or gold layer is applied to the surface of the substrate material using a hot-stamping die for generating an RFID antenna pattern.
What is disadvantageous or problematic about the technique of stamping is that only extremely thin metallic layers within the range of several micrometers may be processed, and that the resulting conductor patterns thus exhibit very poor electric characteristics, such as low conductivity. In addition, stamping operations inherently cannot fabricate any homogenous, smooth contours on the resulting conductor patterns.
In summary, the above-mentioned production techniques are problematic for various reasons. For example, they are very expensive with regard to the multi-stage production process and the variety of process steps, where sometimes only sequential methods are possible and the conductor patterns or RFID antenna patterns produced exhibit poor quality and performance, in particular when using screen printing and the stamping technique. If the etching technique or a an electroplating bath is used, the expenditure and the cost of ensuring the waste-water purification with regard to the wet chemistry also have disadvantageous effects. The production techniques are therefore, in summary, very costly with regard to the fabrication of the conductor patters, in particular for mass production of RFID antenna patterns.
Therefore, because of the above-mentioned problems with regard to very high-effort, costly production methods, RFID systems have so far not been able to become accepted on the market as a mass product in the low-cost segment despite their various possibilities of application and advantages over other wireless identification systems.
Document U.S. Pat. No. 6,618,939 relates to a method for producing resonant tags, a metal foil being stamped out, or punched out, using a thermal adhesion adhesive, on at least one face of the foil into circuit-like molds, and is adhered to a base foil. The method comprises: stamping out parts of metal foils, shaped in a predefined manner, from a metal foil while the latter is guided through a die roll having a stamping blade of a predefined shape, and a transfer roll which is in contact with the die roll and is also employed as a support roll, holding this metal foil portion obtained by the stamping out on the surface of the transfer roll by means of suction holes formed in the transfer roll; and thermally adhering the stamped metal foil portion to the base foil between the transfer roll on its other face and an adhesive roll.